Emerging flux tubes from the solar interior into the atmosphere: effects of non-constant twist

¹ School of Mathematics and Statistics, University of St. Andrews, North Haugh, St. Andrews, Fife, KY16 9SS, UK e-mail: mjm@mssl.ucl.ac.uk
² University College London, Mullard Space Science Laboratory, Holmbury St Mary, Dorking, Surrey, RH5 6NT, UK

Received: 15 October 2007
Accepted: 7 December 2007

Abstract

Context.Observations of large-scale solar emergence events indicate that the magnetic field is already twisted prior to its emergence. However, the nature of twist of the pre-emergence field is largely unknown.

Aims.By testing two different twist profiles for subsurface magnetic flux tubes, we aim to identify any differences in the emergence process of the field and its atmospheric signatures that occur as a result of the twist. Given that differences do occur, future comparisons of these emergence results with observations should reveal specific properties of the twist of the subsurface magnetic field.

Methods.Using a 3D numerical MHD code, we consider a simple stratified model, comprising of one solar interior layer and three overlying atmospheric layers. We set a horizontal, twisted flux tube in the lowest layer and prescribe the central protion along the tube's length to be buoyant so that it will rise towards the surface. We test two different non-constant twist profiles for the tube and perform a parameter study for each profile, looking for differences and similarities during the rise, emergence and expansion stages of the tube's evolution.

Results.We find that, irrespective of the tube's twist profile, if the tube initially has a low tension force then it will experience greater expansion and consequential weakening of its field strength during the rise through the solar interior. Thus, upon reaching the solar surface it will fail to undergo a magnetic buoyancy instability and will not emerge into the atmosphere. For those tubes that do emerge into the atmosphere, there is little distinction between the atmospheric field and few indicators as to the initial twist profile of the tube. In general, tubes with stronger tension forces have a faster growth rate of the magnetic buoyancy instability, while tubes with weaker tension forces expand to a greater degree in the horizontal direction post-emergence. Synthesised magnetograms at the solar surface do vary between the two tested twist profiles but only in cases with initially low tension forces.

Conclusions.Upon emergence, it appears that most of the specific details of the tube's initial twist are lost. The field must initially be sufficiently twisted such that it is able to undergo a magnetic buoyancy instability but such a level of twist subsequently results in the emerging flux having generic atmospheric characteristics. Only in cases with an initially low tension force is it possible to make some distiction post-emergence by considering magnetograms at the solar surface.